Abstract : It is concluded from these studies that the heart and circulation are capable of meeting the demands of oxygen delivery during heavy work at altitudes up to 14,000 ft. There was no evidence of any deleterious effect on the circulatory system nor of any reduced efficiency in the heart's action. Even though maximum performance is reduced at this elevation, it does not appear that the heart and circulation are responsible. Lastly, it can be recommended that gradual ascent to high elevation will be definitely advantageous from the standpoint of the cardiovascular system since it will significantly reduce the work load imposed upon the heart, particularly during the early stages of acclimatization when other problems such as pulmonary edema and altitude sickness are most apt to be prominent.